Mon. Not. R. Astron. Soc. 000,1{11 (2015) Printed 1 October 2018 (MN LATEX style file v2.2) The Local Group as a time machine: studying the high-redshift Universe with nearby galaxies Michael Boylan-Kolchin1?, Daniel R. Weisz2y, Benjamin D. Johnson3, James S. Bullock4, Charlie Conroy3, Alex Fitts1 1Department of Astronomy and Joint Space-Science Institute, University of Maryland, College Park, MD 20742-2421, USA 2Astronomy Department, Box 351580, University of Washington, Seattle, WA, USA 3Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge MA 02138, USA 4Department of Physics and Astronomy, University of California at Irvine, Irvine, CA 92697, USA 1 October 2018 ABSTRACT We infer the UV luminosities of Local Group galaxies at early cosmic times (z ∼ 2 and z ∼ 7) by combining stellar population synthesis modeling with star formation histories derived from deep color-magnitude diagrams constructed from Hubble Space Telescope (HST ) observations. Our analysis provides a basis for understanding high-z galaxies { including those that may be unobservable even with the James Webb Space Telescope (JWST ) { in the context of familiar, well-studied objects in the very low-z Universe. We find that, at the epoch of reionization, all Local Group dwarfs were less luminous than the faintest galaxies detectable in deep HST observations of blank fields. We predict that JWST will observe z ∼ 7 progenitors of galaxies similar to the Large Magellanic Cloud today; however, the HST Frontier Fields initiative may already be observing such galaxies, highlighting the power of gravitational lensing. Consensus reionization models require an extrapolation of the observed blank-field luminosity function at z ≈ 7 by at least two orders of magnitude in order to maintain reionization. This scenario requires the progenitors of the Fornax and Sagittarius dwarf spheroidal galaxies to be contributors to the ionizing background at z ∼ 7. Combined with numerical simulations, our results argue for a break in the UV luminosity function from a faint-end slope of α ∼ −2 at MUV . −13 to α ∼ −1:2 at lower luminosities. Applied to photometric samples at lower redshifts, our analysis suggests that HST observations in lensing fields at z ∼ 2 are capable of probing galaxies with luminosities comparable to the expected progenitor of Fornax. Key words: Local Group { galaxies: evolution { galaxies: high-redshift { cosmology: theory 1 INTRODUCTION even JWST is unlikely to reach the faintest (and most nu- merous) of these sources (see Section 3.2). Faint galaxies below the detection limits of current observa- arXiv:1504.06621v2 [astro-ph.CO] 27 Aug 2015 Resolved-star observations of Local Group galaxies pro- tories are necessary contributors to the ionizing background vide an alternate path for learning about the faintest galax- in the galaxy-dominated models of cosmic reionization that ies at the epoch of reionization, via \archeological" studies are currently favored. Deep-field observations with the Hub- of their descendants. Local galaxies also provide important ble Space Telescope (HST ) only reach ≈ 0:1 L∗(z ∼ 7), benchmarks against which data at a variety of redshifts can while essentially all models of reionization require galax- be compared. Accordingly, observations in the Local Group ies 10 − 1000 times fainter to contribute to the ionizing have long informed our understanding of faint galaxies at background (Alvarez et al. 2012; Kuhlen & Faucher-Gigu`ere early times (Bullock et al. 2000; Freeman & Bland-Hawthorn 2012; Duffy et al. 2014; Robertson et al. 2015; Bouwens et al. 2002; Ricotti & Gnedin 2005; Madau et al. 2008; Bovill & Ri- 2015a). Probing these likely drivers of reionization is a prime cotti 2011; Brown et al. 2012; Ben´ıtez-Llambay et al. 2015). motivation of the James Webb Space Telescope (JWST ), yet The relation between high-z star formation rates (SFRs) and dark matter halo masses for the progenitors ? [email protected] of Local Group dwarfs is of particular interest for under- y Hubble Fellow standing the reionization epoch. Boylan-Kolchin et al.(2014, c 2015 RAS 2 M. Boylan-Kolchin et al. hereafter B14) showed that the requisite inefficient star for- supplement this dataset with SFHs of Leo A (presented in mation in low-mass dark matter halos at z = 0 proves diffi- Cole et al. 2007) and IC 1613 (presented in Skillman et al. cult to reconcile with expected physics of the high-redshift 2014), which are based on newer imaging that includes the Universe in general and reionization models in particular, ancient MSTO. Our sample includes SFHs for all known 5 as these often require relatively efficient star formation in bright (LV > 10 LV; ) MW satellite galaxies with the ex- low-mass halos at early times. Indeed, Madau et al.(2014) ception of Sextans (for which there is no sufficiently deep suggested that the reionization-era progenitors of present- HST imaging), as well as several Local Group dwarf irreg- day dwarf galaxies may have been among the most efficient ular galaxies. SFHs from Local Group galaxies are \non- sites of conversion of gas into stars on galactic scales. Effi- parametric" in the sense that they are the sum of simple cient galaxy formation at high redshift has low-redshift im- stellar populations (e.g., Dolphin 2002) as opposed to im- plications, however: based on counts of remnants of atomic posed analytic prescriptions (e.g., τ models). cooling halos from z = 8 that survive to z = 0 in simulated The SFH of each galaxy is input into the Flexible Stel- Local Groups, B14 argued that the UV luminosity function lar Population Synthesis code (Conroy et al. 2009; Conroy (LF) should break to a shallower value than the observed & Gunn 2010) in order to generate UV and V -band flux α ≈ −2 for galaxies fainter than MUV ∼ −14 (correspond- profiles over time. In both of these steps, we use the Padova 9 ing to halo masses of Mvir . 10 M ). stellar evolution models (Girardi et al. 2002, 2010) and a Weisz et al.(2014b, hereafter W14) further demon- Kroupa(2001) initial mass function (IMF). We assume a strated the power of combining local observations with stel- constant metallicity of 0:2 Z and no internal dust extinc- lar population synthesis models in confronting LFs in the tion (see the discussion in Section 2.3). The latter assump- high-redshift Universe: they showed that one can construct tion appears to be in broad agreement with the apparent LFs that reach several orders of magnitude fainter than is dust-free spectral energy distributions of faint, high-redshift currently observable. The results of W14 indicate that UV galaxies (e.g., Bouwens et al. 2012; Dunlop et al. 2013) and LFs continue without a sharp truncation to much fainter with expectations from simulations (Salvaterra et al. 2011). galaxies [MUV(z ∼ 5) ≈ −5], showing how near-field obser- Finally, in order to generate the simulated fluxes as a func- vations can inform our understanding of galaxies that are tion of redshift, we require the simulated V -band luminosity not directly observable at present. at z = 0 to match observations of Local Group galaxies as In this paper, we combine the approaches of B14 and listed in McConnachie(2012). This normalization accounts W14. Using resolved star formation histories (SFHs), we for the fact that the HST field of view does not always cover model the UV luminosities that Local Group galaxies had the entire spatial extent of a nearby galaxy. at earlier epochs. We compare these to existing HST data We initially generate predicted fluxes assuming the fidu- at z ∼ 2 (from UV dropout galaxies in blank fields and cial SFH, i.e., a constant SFH over each time bin. However, cluster lensing fields) and at z ∼ 7 (from the Ultra-Deep as suggested by a variety of simulations1 (Stinson et al. 2007; Field (HUDF) and the Frontier Fields). As we show below, Ricotti et al. 2008; Governato et al. 2012; Zolotov et al. 2012; this approach holds the promise of both placing the faintest Teyssier et al. 2013; Dom´ınguez et al. 2014; Power et al. observable galaxies at high redshift into a more familiar con- 2014; O~norbe et al. 2015) and observations (e.g., van der text and understanding the possible role of well-studied Lo- Wel et al. 2011; Kauffmann 2014), the SFHs of low-mass cal Group galaxies in high-redshift processes such as cosmic galaxies at high redshift are likely to fluctuate on timescales reionization. (∼ 10-100 Myr) that are shorter than what is directly avail- able from the fossil record (which provides a time resolution of ∼ 10-15% of a given lookback time for CMDs that include the oldest MSTO). 2 MODELING LOCAL GROUP GALAXIES AT To account for short duration bursts of star formation, HIGHER REDSHIFTS we have modified the fiducial SFHs of our Local Group sam- ple to include short duration (∼ 10-100 Myr) bursts. That is, 2.1 Deriving M (z) UV in each time bin, we insert bursts with a specified amplitude To interpret our observations of Local Group galaxies in a and duration that are stochastically spaced in time, with high-redshift context, we follow the methodology described the requirement that the total stellar mass formed in each in W14. This procedure is summarized here; for further de- time bin matches that of the fiducial SFH. For the purpose tails, see W14 and references therein. of this exercise, we have adopted two representative burst We begin with SFHs derived from resolved star color- models: 200 Myr duration with amplification factor of 5 and magnitude diagrams (CMDs) constructed from HST imag- 20 Myr duration with amplification factor of 20. In both ing. The majority of the SFHs we use are based on analysis cases, we assume that 80% of the star formation occurs in of archival HST /WFPC2 imaging; these data, the SFHs, the bursting phase.